Method, apparatus, and system for handover to circuit switched domain

ABSTRACT

Embodiments of the present invention provide a method, apparatus, and system for a handover to a circuit switched domain. The method includes: selecting an MSC capable of both CSFB and SRVCC for a UE; and selecting, if a location service of a CS domain triggers an SRVCC procedure for the UE when the UE has a VOIP service in progress, the MSC capable of both CSFB and SRVCC in the SRVCC procedure, so that the UE, after being handed over to the CS domain, accesses the MSC and implements the location service of the CS domain. The apparatus includes: a first selecting module and a second selecting module. The system includes a UE and the apparatus. The embodiments of the present invention enable the UE with the VOIP service in progress to be correctly handed over to the CS domain and implement the location service.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/593,905 filed on Jan. 9, 2015, which is a continuation ofInternational Application No. PCT/CN2012/078630 filed on Jul. 13, 2012.All of the afore-mentioned patent applications are hereby incorporatedby reference in their entireties.

TECHNICAL FIELD

The present invention relates to the field of communicationtechnologies, and in particular, to a method, apparatus, and system fora handover to a circuit switched domain.

BACKGROUND

The 3GPP (3rd Generation Partnership Project) standards organization hasnow proposed a new-generation evolved network architecture, in which acore network part is an SAE (SystemArchitecture Evolution) network andan access network part is an LTE (Long Term Evolution) network, alsocalled an E-UTRAN (Evolved Universal Terrestrial Radio Access Network).The E-UTRAN is formed by eNBs (evolved NodeBs). The SAE network isformed by two parts: MME (Mobility Management Entity) and SAE gateway.SAE gateways include a serving gateway (Serving Gateway) and a PDNGateway (Packet Data Network Gateway). An MME is a node on a controlplane of the core network part and is in charge of managing a context ofa UE (User Equipment) and bearers on a user plane. A serving gatewaytakes care of transmitting subscriber service data, and a PDN Gateway isin charge of transmitting subscriber signaling data.

When a UE is performing a VOIP (Voice over Internet Protocol) service ina PS (Packet Switched) domain of an SAE/LTE network, if a CS MT-LR(Mobile Terminated Location Request) procedure is triggered due to alocation service of a CS (Circuit Switched) domain, an SRVCC (SingleRadio Voice Call Continuity) procedure needs to be executed to hand overthe UE back to the CS domain. After the UE is handed over back to the CSdomain, an MSC for previous combined attach notifies the UE of providinglocation information. In this scenario, if the selected MSC does nothave an SRVCC capability, the UE is unable to complete the locationservice of the CS domain.

SUMMARY

Embodiments of the present invention provide a method, apparatus, andsystem for a handover to a circuit switched domain, so that a UE with anongoing VOIP service is able to be correctly handed over to a CS domain.The technical solutions are as follows:

According to an aspect, a method for a handover to a circuit switcheddomain is provided, including:

selecting a mobile switching center MSC capable of both circuit switchedfallback CSFB and single radio voice call continuity SRVCC for a userequipment UE; and

selecting the MSC capable of both CSFB and SRVCC for the UE in an SRVCCprocedure if a location service of a circuit switched CS domain triggersthe SRVCC procedure for the UE during an ongoing voice over InternetProtocol VOIP service of the UE, so that the UE, after being handed overto the CS domain, accesses the MSC and implements the location serviceof the CS domain.

According to another aspect, an apparatus for a handover to a circuitswitched domain is provided, including:

a first selecting module, configured to select a mobile switching centerMSC capable of both circuit switched fallback CSFB and single radiovoice call continuity SRVCC for a user equipment UE; and

a second selecting module, configured to: if a location service of acircuit switched CS domain triggers an SRVCC procedure for the UE whenthe UE has voice over Internet Protocol VOIP service in progress, selectthe MSC that is selected by the first selecting module and is capable ofboth CSFB and SRVCC for the UE in the SRVCC procedure, so that the UE,after being handed over to the CS domain, accesses the MSC andimplements the location service of the CS domain.

According to still another aspect, a system for a handover to a circuitswitched domain is provided, including a UE and the apparatus for ahandover to a circuit switched domain;

the UE is configured to: if a location service of a circuit switched CSdomain triggers a single radio voice call continuity SRVCC procedurewhen the UE has a voice over Internet Protocol VOIP service in progress,hand over to the CS domain to access a mobile switching center MSC thatis selected by the apparatus and is capable of both circuit switchedfallback CSFB and SRVCC, and implement the location service of the CSdomain.

Beneficial effects of technical solutions provided by the embodiments ofthe present invention are as follows:

an MSC capable of both CSFB and SRVCC is selected for a UE, and if alocation service of a CS domain triggers an SRVCC procedure for the UEwhen a VOIP service of the UE is in progress, the MSC capable of bothCSFB and SRVCC is selected in the SRVCC procedure. In this way, it isensured that the UE can correctly access the CSFB enabled MSC toimplement the location service in the CS domain while the SRVCCprocedure can continue normally, thereby solving a problem in the priorart where a UE is incapable of implementing a location service of a CSdomain when an MME selects an SRVCC-incapable MSC in combined attach.

BRIEF DESCRIPTION OF THE DRAWINGS

To describe the technical solutions in the embodiments of the presentinvention more clearly, the following briefly introduces theaccompanying drawings required for describing the embodiments.Apparently, the accompanying drawings in the following description showmerely some embodiments of the present invention, and a person ofordinary skill in the art may still derive other drawings from theseaccompanying drawings without creative efforts.

FIG. 1 is a schematic diagram of a network architecture according to anembodiment of the present invention;

FIG. 2 is a flowchart of a method for a handover to a circuit switcheddomain according to an embodiment of the present invention;

FIG. 3 is a flowchart of a method for a handover to a circuit switcheddomain according to another embodiment of the present invention;

FIG. 4 is a schematic diagram of a combined attach procedure accordingto still another embodiment of the present invention;

FIG. 5 is a schematic diagram of an SRVCC procedure triggered by alocation service according to yet another embodiment of the presentinvention;

FIG. 6 is a flowchart of a method for a handover to a circuit switcheddomain according to another embodiment of the present invention;

FIG. 7 is a schematic diagram of a combined tracking area updateprocedure according to still another embodiment of the presentinvention;

FIG. 8 is an architecture diagram of an apparatus for a handover to acircuit switched domain according to yet another embodiment of thepresent invention; and

FIG. 9 is an architecture diagram of a system for a handover to acircuit switched domain according to another embodiment of the presentinvention.

DETAILED DESCRIPTION

To make the objectives, technical solutions, and advantages of theembodiment of the present invention clearer, the following furtherdescribes the embodiment of the present invention in detail withreference to the accompanying drawings. Apparently, the describedembodiments are merely a part rather than all of the embodiments of thepresent invention. All other embodiments obtained by a person ofordinary skill in the art based on the embodiments of the presentinvention without creative efforts shall fall within the protectionscope of the embodiment of the present invention.

Refer to FIG. 1, which is a schematic diagram of a network architectureapplied in an embodiment of the present invention. The upper part inFIG. 1 is a 2G/3G network. The 2G access network is a GERAN (GSM EDGERadio Access Network), and the 3G access network is a UTRAN (UMTSTerrestrial Radio Access Network). Devices such as an MSC are includedin the 2G/3G network. In the embodiments of the present invention, anMSC may also be called an MSC server, and for ease of description, onlythe concept of MSC is applied in the remainder of this application,unless otherwise stated. In the 2G/3G network, a voice service isgenerally borne by a CS domain and a data service is generally borne bya PS domain. The middle part in FIG. 1 is an SAE network and the lowerpart is an LTE network, including devices such as an SGSN, an MME, anHSS (Home Subscriber Server) and a Serving/PDN GW. The SAE/LTE networkserves as a packet switched system over a PS domain. A voice service canonly be borne by a PS domain for transmission, and the voice service iscontrolled by an IMS (IP Multimedia Subsystem) over the IP-based bearernetwork. When a UE is located in a PS domain, an MME provides servicesfor the UE. When the UE is located in a CS domain, an MSC providesservices for the UE. In the embodiments of the present invention, ahandover of a UE to a CS domain means that the UE is handed over fromthe PS domain to the CS domain when the UE has a VOIP service inprogress.

The embodiments of the present invention further relate to a CSFBtechnology. In the CSFB technology, the UE performs combined attach(Combined Attach) to fall back from a PS domain of LTE to a CS domain ofa 2G/3G network. In the combined attach procedure, an MME selects an MSCfor the UE so that the MSC provides services for the UE when the UEfalls back to the CS domain.

Referring to FIG. 2, a method for a handover to a circuit switcheddomain is provided according to an embodiment of the present invention,including:

201: Select a mobile switching center MSC capable of both circuitswitched fallback CSFB and single radio voice call continuity SRVCC fora user equipment UE.

202: Select, if a location service of a circuit switched CS domaintriggers an SRVCC procedure for the UE when the UE has a voice overInternet Protocol VOIP service in progress, the MSC capable of both CSFBand SRVCC for the UE in the SRVCC procedure, so that the UE, after beinghanded over to the CS domain, accesses the MSC and implements thelocation service of the CS domain.

The foregoing method according to this embodiment may be executed by anMME.

In the foregoing method provided by this embodiment, an MSC capable ofboth CSFB and SRVCC is selected for a UE, and if a location service of aCS domain triggers an SRVCC procedure for the UE when a VOIP service ofthe UE is in progress, the MSC capable of both CSFB and SRVCC isselected in the SRVCC procedure. In this way, it is ensured that the UEcan correctly access the CSFB-capable MSC while the SRVCC procedure cancontinue normally, thereby solving a problem in the prior art where a UEis incapable of implementing a location service of a CS domain when anMME selects an SRVCC-incapable MSC in combined attach.

Referring to FIG. 3, a method for a handover to a circuit switcheddomain is provided according to another embodiment of the presentinvention. The following description is based on an example in which anMSC capable of both CSFB and SRVCC is selected by an MME in a combinedattach procedure. The method specifically includes:

301: In a combined attach procedure of a UE, an MME selects a first MSCcapable of both CSFB and SRVCC for the UE.

Generally, the UE performs a combined attach procedure when the UE ispowered on. In the combined attach procedure, the UE is first attachedto the MME of a PS domain, and then the MME selects an MSC for the UEand sends an update location request to the MSC, so that the UE is thenhanded over to the CS domain and the MSC provides services for the UE.The procedure in which the UE is attached to the MME and attached to theMSC is a combined attach procedure. In this embodiment, the MME selectsthe first MSC capable of both CSFB and SRVCC for the UE, so that the UEcan be correctly handed over to the CS domain when a subsequent SRVCCprocedure is performed.

302: If a location service of the CS domain triggers an SRVCC procedurefor the UE during an ongoing VOIP service of the UE, the MME selects thefirst MSC in the SRVCC procedure, so that the UE, after being handedover to the CS domain, accesses the first MSC and implements thelocation service of the CS domain.

The VOIP service is a service of the UE in the PS domain. The locationservice of the CS domain is a CS-domain mobile terminated locationservice CS MT-LR that is triggered by the location service of the UE inthe CS domain. For example, if the UE serves as a called party toreceive a call from the CS domain, the MSC pages the UE in the CS domainand requests the UE to provide its location information.

The location service of the CS domain triggers the SRVCC procedure, andby means of the SRVCC procedure, the UE can be handed over from the PSdomain to the CS domain. In this procedure, the MME selects an MSC forthe UE so that the MSC provides services for the UE after the UE ishanded over to the CS domain. In this embodiment, the MME selects, inthe SRVCC procedure, the MSC that is the same as the MSC selected in thecombined attach procedure. Because the MSC is capable of both CSFB andSRVCC, it is ensured that the UE can be correctly handed over to the CSdomain. When the UE has accessed the first MSC, it means that the UE hasalready been handed over to the CS domain, and in this case, the MSC canprovide services for the UE.

Refer to FIG. 4. In this embodiment, the combined attach procedure instep 301 may specifically include:

401: A UE sends a combined attach request message to an MME to requestperforming combined attach, where the message includes an indicationrepresentative of combined attach.

402: After receiving the combined attach request message, the MMEselects an MSC capable of both CSFB and SRVCC for the UE.

403: The MME sends a location update request message to the selected MSCto request a location update.

404: After receiving the location update request message sent by theMME, the MSC interacts with an HSS to perform a location update for theUE towards the CS domain.

405: The MSC sends a location update accept message to the MME.

406: After receiving the location update accept message, the MME sends acombined attach accept message to the UE. The combined attach procedureends.

Through the foregoing combined attach procedure, an MME selects an MSCcapable of both CSFB and SRVCC for a UE to avoid a problem where asubsequent SRVCC procedure cannot continue normally because the selectedMSC does not support SRVCC. Meanwhile, the MSC further supports CSFB, sothat the procedure of handing over the UE to a CS domain can beperformed normally.

Refer to FIG. 5. In this embodiment, the SRVCC procedure in step 302which is triggered by a location service of a CS domain for a UE with anongoing VOIP service may specifically include:

501: The UE is implementing the VOIP service in a PS domain.

502: Because the location service of the CS domain triggers a CS-domainmobile terminated location service CS MT-LR, an MSC receives a providesubscriber location message, which is a request for providing locationinformation.

502 a: The MSC sends a paging request message to an MME. The MME sends aCS paging notification message to the UE to notify the UE that this is apaging due to the location service, and sends a service request messageto the MSC.

502 b: The UE sends an extended service request message to the MME.

502 c: If the MME determines that the UE rejects the current CS MT-LRrequest, the MME sends a CS Paging Reject message to the MSC.

502 d: If the MME determines that the UE accepts the current CS MT-LRrequest, the MME sends a message to notify an eNodeB, where the messageincludes a CSFB indicator.

503: After receiving the message, the eNodeB returns a response messageto the MME and requests the UE to measure a 2G/3G network according tothe CSFB indicator.

504: The eNodeB determines and triggers execution of the SRVCC procedureand hands over the UE to the 2G/3G network. In the SRVCC procedure, theMME selects an MSC that is the same as the MSC selected in the foregoingcombined attach procedure. Because the MSC is capable of both CSFB andSRVCC, it is ensured that the MSC to which the UE is connected after theUE is handed over to the CS domain is the MSC in step 502 a. In thiscase, the CS location service can be continued.

505: After the handover is complete for the UE, the MSC sends an LCS(Location Notification Invoke) message to the UE.

506: After receiving the location notification invoke message, the UEimplements the location service and provides its location information tothe MSC. The procedure ends.

In the foregoing method according to this embodiment, an MSC capable ofboth CSFB and SRVCC is selected by an MME for a UE, and if a locationservice of a CS domain triggers an SRVCC procedure for the UE when theUE has a VOIP service in progress, the MSC capable of both CSFB andSRVCC is selected in the SRVCC procedure. In this way, it is ensuredthat the UE can correctly access the CSFB-capable MSC while the SRVCCprocedure can continue normally, thereby solving a problem in the priorart where a UE is incapable of implementing a location service of a CSdomain when an MME selects an SRVCC-incapable MSC in combined attach.

Refer to FIG. 6. A method for a handover to a circuit switched domain isprovided in another embodiment of the present invention. The followingdescription is based on an example in which an MSC capable of both CSFBand SRVCC is selected by an MME in a combined tracking area updateprocedure. The method specifically includes:

601: When a UE has a VOIP service in progress, an MME sends to the UE anIMSI (International Mobile Subscriber Identity) detach messageinstructing the UE to detach from an MSC selected in a combined attachprocedure and perform a Combined TAU (Combined Tracking Area Update)procedure.

Before the UE implements the VOIP service, the MME has already selectedan MSC for the UE through a combined attach procedure that has beenperformed. Specifically, the MME may select an MSC according to anexisting procedure.

In this embodiment, the MME detects whether the UE is implementing theVOIP service. When it is detected that the UE is implementing the VOIPservice, a message is sent to the UE to instruct the UE to detach fromthe MSC selected in the combined attach procedure, so as to release anassociation relationship between the UE and the MSC, and instruct the UEto initiate a combined tracking area update procedure, so as to performcombined attach for the UE again. The method for the MME to detectwhether the UE is implementing the VOIP service specifically includes:According to a bearer established for the UE, the MME determines whethera QCI (QoS Class Identify) of the bearer is equal to 1. If the QCI isequal to 1, the MME determines that the UE is implementing the VOIPservice. If the QCI is not equal to 1, the MME determines that the UE isnot implementing the VOIP service.

602: In a combined tracking area update procedure, the MME selects asecond MSC capable of both CSFB and SRVCC for the UE.

In the combined tracking area update procedure, the MME performscombined attach for the UE again. In this way, an MSC capable of bothCSFB and SRVCC can be selected, so that the UE may be correctly handedover to a CS domain in a subsequent procedure.

603: If a location service of a CS domain triggers an SRVCC procedurefor the UE during the ongoing VOIP service of the UE, the MME selectsthe second MSC in the SRVCC procedure, so that the UE, after beinghanded over to the CS domain, accesses the second MSC and implements thelocation service of the CS domain.

Refer to FIG. 7. In this embodiment, the combined tracking area updateprocedure in step 601 may specifically include:

701: A UE performs a combined attach procedure, and during thisprocedure, the UE is attached to an MME and attached to an MSC.

702: The UE implements a VOIP service.

703: After detecting that the UE is implementing the VOIP service, theMME sends an IMSI detach message to the UE to instruct the UE to detachfrom the foregoing MSC and perform a combined tracking area updateprocedure.

When implementing the VOIP service, the UE performs a bearer setupprocedure. In the bearer setup procedure, the MME makes a determinationaccording to a QCI (QoS Class Identifier) in the received message. If avalue of the QCI is 1, it indicates that the UE is implementing the VOIPservice. If a value of the QCI is 0, it indicates that the UE is notimplementing the VOIP service.

704: After receiving the IMSI detach message, the UE sends a CombinedTAU message to the MME to initiate the combined tracking area updateprocedure.

705: The MME reselects an MSC capable of both CSFB and SRVCC.

706: The MME sends a location update request message to the reselectedMSC to request a location update.

707: After receiving the location update request message sent by theMME, the MSC interacts with an HSS to perform a location update for theUE towards the CS domain.

708: The MSC sends a location update accept message to the MME.

709: After receiving the location update accept message, the MME sends acombined tracking area update accept message to the UE. The combinedtracking area update procedure ends.

An SRVCC procedure triggered based on a location service in the CSdomain after the UE performs the tracking area update procedure issimilar to the procedure illustrated in FIG. 5, and details are notdescribed herein again. A difference is that, in the SRVCC procedure instep 504, the MME selects an MSC that is the same as the MSC selected inthe foregoing combined attach procedure, whereas in this SRVCCprocedure, the MME selects an MSC that is the same as the MSC selectedin the foregoing combined tracking area update procedure. By means ofthe foregoing procedure, it is ensured that the UE can be correctlyhanded over to the CS domain.

In the foregoing method provided in this embodiment, an MSC capable ofboth CSFB and SRVCC is selected for a UE by an MME, and if a locationservice of a CS domain triggers an SRVCC procedure for the UE during anongoing VOIP service of the UE, the MSC capable of both CSFB and SRVCCis selected in the SRVCC procedure. In this way, it is ensured that theUE can correctly access the CSFB-capable MSC, thereby solving a problemin the prior art where a UE is incapable of implementing a locationservice of a CS domain when an MME selects an SRVCC-incapable MSC incombined attach.

Referring to FIG. 8, an apparatus for a handover to a circuit switcheddomain is provided in yet another embodiment of the present invention,including:

a first selecting module 801, configured to select a mobile switchingcenter MSC capable of both circuit switched fallback CSFB and singleradio voice call continuity SRVCC for a user equipment UE; and

a second selecting module 802, configured to: if a location service of acircuit switched CS domain triggers an SRVCC procedure for the UE whenthe UE has a VOIP service in progress, select the MSC that is selectedby the first selecting module 801 and is capable of both CSFB and SRVCCfor the UE in the SRVCC procedure, so that the UE, after being handedover to the CS domain, accesses the MSC and implements the locationservice of the CS domain.

In one aspect, the first selecting module 801 may be specificallyconfigured to select a first MSC capable of both CSFB and SRVCC for theUE in a combined attach procedure of the UE.

Correspondingly, the second selecting module 802 may be specificallyconfigured to: if the location service of the CS domain triggers anSRVCC procedure for the UE when the UE has the VOIP service in progress,select the first MSC which is selected by the first selecting module 801in the SRVCC procedure, so that the UE, after being handed over to theCS domain, accesses the first MSC and implements the location service ofthe CS domain.

In another aspect, the first selecting module 801 may be specificallyconfigured to: when the UE is implementing the VOIP service, send to theUE an IMSI detach message instructing the UE to detach from an MSCselected in a combined attach procedure and perform a combined trackingarea update procedure. In the combined tracking area update procedure, asecond MSC capable of both CSFB and SRVCC is selected for the UE.

Correspondingly, the second selecting module 802 may be specificallyconfigured to: if the location service of the CS domain triggers anSRVCC procedure for the UE when the UE has the VOIP service in progress,select the second MSC which is selected by the first selecting module801 in the SRVCC procedure, so that the UE, after being handed over tothe CS domain, accesses the second MSC and implements the locationservice of the CS domain.

The foregoing apparatus provided in this embodiment may be specificallyan MME.

The foregoing apparatus provided in this embodiment is configured toselect an MSC capable of both CSFB and SRVCC for a UE, and if a locationservice of a CS domain triggers an SRVCC procedure for the UE during anongoing VOIP service of the UE, select the MSC capable of both CSFB andSRVCC in the SRVCC procedure. In this way, it is ensured that the UE cancorrectly access the CSFB-capable MSC while the SRVCC procedure cancontinue normally, thereby solving a problem in the prior art where a UEis incapable of implementing a location service of a CS domain when anMME selects an SRVCC-incapable MSC in combined attach.

Referring to FIG. 9, a system for a handover to a circuit switcheddomain is provided in another embodiment of the present invention,including a UE 901 and an apparatus 902 for a handover to a circuitswitched domain;

the UE 901 is configured to: if a location service of a circuit switchedCS domain triggers an SRVCC procedure for the UE when the UE has a VOIPservice in progress, hand over to the CS domain to access the MSC thatis selected by the apparatus 902 and capable of both CSFB and SRVCC, andimplement the location service of the CS domain.

Functions of the apparatus 902 are the same as functions of theapparatus provided in the foregoing embodiment.

The system provided in this embodiment may perform the method providedin any one of the foregoing method embodiments. For a detailedprocedure, refer to the description in the method embodiments. Detailsare not described herein again.

In the foregoing system provided in this embodiment, the apparatusselects an MSC capable of both CSFB and SRVCC for a UE, and if alocation service of a CS domain triggers an SRVCC procedure for the UEduring an ongoing VOIP service of the UE, selects the MSC capable ofboth CSFB and SRVCC in the SRVCC procedure. In this way, it is ensuredthat the UE can correctly access the CSFB-capable MSC while the SRVCCprocedure can continue normally, thereby solving a problem in the priorart where a UE is incapable of implementing a location service of a CSdomain when an MME selects an SRVCC-incapable MSC in combined attach.

It should be noted that the units included in the foregoing userequipment embodiments are divided according to functional logic, but theembodiment of the present invention is not meant to be limited to theforegoing division, so long as the corresponding functions can beimplemented. In addition, a specific name of each functional unit is forease of distinguishing each other and is not intended to limit theprotection scope of the embodiment of the present invention.

In addition, a person of ordinary skill in the art may understand thatall or a part of the steps of the method embodiments may be implementedby a program instructing relevant hardware. The program may be stored ina computer readable storage medium. The storage medium may include aread-only memory, a magnetic disk, or an optical disc.

The foregoing descriptions are merely exemplary specific implementationsof the embodiment of the present invention, but are not intended tolimit the protection scope of the embodiment of the present invention.Any variation or replacement readily figured out by a person skilled inthe art within the technical scope disclosed in the embodiment of thepresent invention shall fall within the protection scope of theembodiment of the present invention. Therefore, the protection scope ofthe embodiment of the present invention shall be subject to theprotection scope of the claims.

What is claimed is:
 1. A method for a handover to a circuit switcheddomain, the method comprising: selecting, by a mobility managemententity (MME), a mobile switching center (MSC) capable of supporting bothcircuit switched fallback (CSFB) and single radio voice call continuity(SRVCC) for a user equipment (UE) from at least two MCSs in a combinedtracking area update procedure of the UE, wherein the at least two MSCscomprise the MSC capable of supporting both CSFB and SRVCC and an MSCnot supporting SRVCC; and when a location service of a circuit switched(CS) domain triggers an SRVCC procedure for the UE when the UE has aVoice over Internet Protocol (VOIP) service in progress, selecting, bythe MME, the MSC selected in the combined tracking area updateprocedure.
 2. An apparatus for a handover to a circuit switched domain,the apparatus comprising: a memory, configured to store a programinstruction and data; and a processor, configured to: select a mobileswitching center (MSC) capable of supporting both circuit switchedfallback (CSFB) and single radio voice call continuity (SRVCC) for auser equipment (UE) from at least two MCSs in a combined tracking areaupdate procedure of the UE, wherein the at least two MSCs comprise theMSC capable of supporting both CSFB and SRVCC and an MSC not supportingSRVCC, and when a location service of a circuit switched (CS) domaintriggers an SRVCC procedure for the UE when the UE has a voice overInternet Protocol (VOIP) service in progress, select the MSC selected inthe combined tracking area update procedure.
 3. A non-transitorycomputer readable storage medium storing program codes for use by amobility management entity (MME) for a handover to a circuit switcheddomain, wherein the program codes comprise instructions for: selecting amobile switching center (MSC) capable of supporting both circuitswitched fallback (CSFB) and single radio voice call continuity (SRVCC)for a user equipment (UE) from at least two MCSs in a combined trackingarea update procedure of the UE, wherein the at least two MSCs comprisethe MSC capable of supporting both CSFB and SRVCC and an MSC notsupporting SRVCC; and when a location service of a circuit switched (CS)domain triggers an SRVCC procedure for the UE when the UE has a voiceover Internet Protocol (VOIP) service in progress, selecting the MSCselected in the combined tracking area update procedure.